The present invention relates to power supplies for high intensity arc discharge (HIAD) lamps and more specifically to circuits which integrate the complete operational control of such lamps over extended dynamic ranges.
The design of HIAD lamps involves many variables: arc length, bore diameter, electrode composition, fill gas, gas pressure, etc. Specific application and technological requirements will dictate which variables are selected for a given HIAD lamp, and thus establish its power and performance characteristics. Indeed, new industrial thermal and radiant processing technologies are emerging which require power supplies that can fully utilize the power and performance curves of state of the art HIAD lamps which may operate at power levels in excess of 40,000 watts.
As an example, one area in which precise, variable, high power lamp control is essential is that of thermal processing of semiconductor wafers. Most existing systems use an array of 10 or more filament lamps to heat such wafers. However, because of the large thermal mass of the filament lamps, they take a comparatively long time to heat wafers up to a given temperature. This poor response in shaping the wafer's time-temperature profile can lead to process problems. Additional process difficulties arise because an array of filament lamps is required to achieve the power levels necessary for high temperature processing. Each lamp may have slightly unique characteristics and may age differently, resulting in both process uniformity and reliability problems.
A single HIAD lamp can be used for thermal processing of semiconductor wafers and has the advantage of reaching temperature very quickly, thus providing more precise wafer time-temperature profiles. However, a power supply is required which can turn on a HIAD lamp with repeatable precision as well as vary lamp power quickly and accurately from an ultra-low power DC "simmer" mode (less than 400 watts) to a high power AC process mode (40,000 watts or more). The present invention incorporates these advantages and can address similar HIAD thermal and radiant processing requirements in other industries such as plastics, ceramics, and stage lighting to name a few.
An arc lamp is typically turned on by first charging a capacitive boost circuit and then starting the lamp with an igniter to provide a high voltage pulse across the electrodes. Typically, a timing circuit is used so that the igniter is switched on a predetermined amount of time after the boost capacitors start charging. This amount of time is estimated to be sufficient to provide the boost energy required. Often, several start attempts will be necessary in order to get a proper voltage pulse to start the lamp.
Once started, some embodiments then rectify AC line voltage to produce DC voltage which is then applied to a switching bridge to supply a pulsed voltage across the lamp. The bridge may be an SCR (Silicon Controlled Rectifier) switching bridge with an inductor in the bridge or in the circuit immediately after the bridge. The average voltage applied to the load is varied by controlling the pulse width with the bridge. Such a supply can only operate an arc lamp over a limited range because at low power the decreasing width of the pulse modulation causes the voltage to drop off to zero between pulses. This can cause the arc lamp to extinguish, and thus low power operation is not possible. AC operation is required for high power arc lamp operation in order to supply the large currents needed.
U.S. Pat. No. 4,412,156 to Ota discloses an AC power supply for a metal halide discharge lamp which includes a main switch, a commutator and power feedback. The circuit disclosed is designed for AC operation only at a fixed power level. Another AC power supply for a metal halide lamp is shown in U.S. Pat. No. 3,999,100 to Dendy et al. Here again, a fixed lamp power is used, and a power feedback error signal is used to control the switching to provide a constant power output.
A DC lamp power supply is shown in U.S. Pat. No. 4,240,009 to Paul. Again, power feedback is used to maintain a fixed power level. A capacitor is charged to provide the high voltage pulse needed to start the lamp, and circuitry is provided to repeat application of the pulse until the lamp starts. Another DC lamp power supply is shown in U.S. Pat. No. 4,399,392 to Buhrer.
Difficulties arise for a power supply when an arc lamp is operated over a wide range of power levels due to the characteristics of the arc lamp impedance. The power load line of a typical arc lamp (see FIG. 2) shows that at low power, a high voltage is required, with the voltage level dropping as the power increases. The voltage level decreases and levels off as power increases, then increases again at higher power levels, typically above 500 watts. In some applications, such as doing thermal processing of semiconductor wafers, a power supply is needed which can provide the power requirements of an arc lamp over a wide range of power levels.